1. Field of the Invention
The invention relates in general to electrical insulators and in particular to fabric reinforced insulators.
2. Description of the Prior Art
Porcelain and other electrically suitable materials for insulators that have enough mechanical strength to support themselves and the attached power lines are brittle and subject to breakage, often with explosive force. Since the insulator structures are quite large for high voltage apparatus, the potential energy of pressurized insulating gas that may be contained in the interior of the insulator is quite high. Damage to the brittle pressurized porcelain due to vandalism, accidental contact, or electrical puncture can result in catastrophic explosion of the insulator. Insulator fragments may be blown significant distances, endangering personnel as well as adjacent equipment. Cast polymeric resin insulator weather casings have been used for some applications but the high strength resins are typically nearly as brittle as porcelain--both materials are highly breakable and shatter-prone. Prior art solutions to the above dangerous conditions include putting an insulating tube over the installed insulator, and using more expensive resin materials for greater strength. An insulating tube requires additional assembly and may electrically interfere with the insulator. The higher priced resin materials are an improvement; however, known resin materials that are suitable electrically for insulator applications have relatively low tensile strength and are weak and brittle. Casting electrical insulators from fluid uncured resins allows the encapsulation of reinforcing materials such as glass fiber fabrics and other non-metallic fibers. Three problems are inherent with encapsulating reinforcing materials to increase the strength of resin insulators. The first is that a large amount of fabric is necessary to raise the strength of the finished product appreciably, thereby increasing the size of the insulator and increasing cost. If only one or two layers of a reinforcing fabric such as glass fibers are used, the fabric will rip upon fracture of the resin material and the insulator may shatter or explode just as with non-reinforced insulators. A second problem is the location and support of the fabric in the proper position during the filling of the mold with liquid resin and the gelling of the resin. This was accomplished in the prior art by means of support projections within the casting mold, with the fabric being located and held in place by the special projections. Since the tubular fabric members are firmly held by the projections in the mold, they maintained their preplaced positions as the resin system was introduced into the mold and during the subsequent curing of the cast resin insulation system. When the solidified insulator was removed from the mold, the grooves therein caused by the anchoring projections of the mold were filled with solid resin insulation to prevent the fabric members from being in contact with air. This last step was taken to overcome the third problem with encapsulating reinforcing fabrics in cast resinous insulators--a substantial covering of the outside layer of fabric with resin is necessary to provide a smooth exterior peripheral surface. The exterior of the finished insulator should present a smooth, self-cleaning, non-tracking, weather-resistant surface to the outside environment the insulator may be subjected to. Accordingly, it would be desirable to produce a fabric reinforced cast resin insulator that would limit displacement of fragments of the insulator weather casing during catastrophic failure of the insulator. Further it would be desirable if the cast resin insulator could accomplish this without encapsulating a large amount of fabric reinforcement layers so that the finished insulator would be both cost effective and of comparable size with non-reinforced insulators. It would also be desirable if the fabric reinforcement material were designed for ease of location and support during the filling of the mold with liquid resin and the gelling of the resin. Further, it would be desirable that the finished product have a thick resin covering on the outside periphery of the fabric to insure a smooth outer peripheral surface.